Motor

ABSTRACT

A motor includes a stator, a rotor, a case, and back-surface magnet portions. The rotor has a first rotor core, a second rotor core and a field magnet. Each of the first and second rotor cores has a core base and claw-shaped magnetic poles. The field magnet is sandwiched between the first rotor core and the second rotor core and causes the claw-shaped magnetic poles of the first rotor core and the second rotor core to function as different magnetic poles. The back-surface magnet portions include a second and a first back-surface magnet portions respectively provided on the back surfaces of the claw-shaped magnetic poles of the second rotor core and the first rotor core. Size of the second back-surface magnet portion differs from size of the first back-surface magnet portion are different from each other.

RELATED APPLICATIONS

This present application claims priority to Japanese Patent Application No. 2014-083713 filed Apr. 15, 2014, Japanese Patent Application No. 2014-203193 filed Oct. 1, 2014, and Japanese Patent Application No. 2014-213674 filed Oct. 20, 2014 said priority applications being each fully incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a motor accommodating a stator and a rotor in a case.

BACKGROUND OF THE INVENTION

Conventionally, as a rotor used in a motor, a rotor having a Lundell structure with a so-called permanent magnet field disclosed in Japanese Patent Laid-Open Publication No. 2012-115085 and Japanese Utility Model Laid-Open Publication No. 5-43749 is known. Such a rotor includes two rotor cores combined with each other and a field magnet arranged between the two rotor cores. Each of the rotor cores has a plurality of claw-shaped magnetic poles aligned in a peripheral direction. The field magnet causes the claw-shaped magnetic poles to function as alternately different magnetic poles.

Moreover, in the rotor disclosed in Japanese Patent Laid-Open Publication No. 2012-115085, in order to increase an output of the motor, a back-surface magnet for rectifying a magnetic path is arranged on a back surface of the claw-shaped magnetic pole. Moreover, in this rotor, in order to increase the output of the motor, an inter-pole magnet for rectifying the magnetic path is arranged between the alternately arranged claw-shaped magnetic poles. Such a motor is accommodated in a case having a yoke housing having a cylindrical shape with a bottom and an end frame provided on one end of the yoke housing.

In the motor as above, the yoke housing which is a magnetic body is located on one end surface side in an axial direction of the rotor, and the end frame made of a resin is located on the other end surface side in the axial direction of the rotor. In this case, a part of a magnetic flux from the field magnet of the rotor leaks to the case (yoke housing), and there is a concern that a magnetic flux amount (magnetic balance) is unbalanced between the N-pole and the S-pole. Moreover, in this case, detent torque becomes unbalanced, and there is a concern that it causes deterioration of sound and vibration. Furthermore, in this case, a part of the magnetic flux from the field magnet of the rotor leaks to the case (yoke housing) and there is a concern that it causes deterioration of output characteristics.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a motor which can make magnetic balance favorable by adjusting a leakage flux. A second object is to provide a motor which can make a balance of detent torque favorable. A third object is to provide a motor which can suppress the leakage flux and improve the output characteristics.

To achieve the above object, first aspect of the present invention is a motor including a stator, a rotor, a case, and a plurality of back-surface magnet portions. The stator has a stator core and a winding. The rotor has a first rotor core, a second rotor core, and a field magnet. Each of the first and second rotor cores has a substantially disc-shaped core base and a plurality of claw-shaped magnetic poles provided on an outer peripheral portion of the core base along a peripheral direction. The field magnet is sandwiched between the first rotor core and the second rotor core in an axial direction. The field magnet is configured such that the claw-shaped magnetic pole of the first rotor core and the claw-shaped magnetic pole of the second rotor core function as magnetic poles different from each other in a sandwiched state. The case is cylindrical with a bottom. The case has a yoke housing which is a magnetic body and a lid portion closing an opening portion of the yoke housing. The yoke housing accommodates the stator and the rotor. The plurality of back-surface magnet portions is arranged in a gap formed on a back surface of the claw-shaped magnetic pole. The plurality of back-surface magnet portions include a second back-surface magnet portion provided on the back surface of the claw-shaped magnetic pole of the second rotor core and a first back-surface magnet portion provided on the back surface of the claw-shaped magnetic pole of the first rotor core. A size of the second back-surface magnet portion and a size of the first back-surface magnet portion are different from each other.

A second aspect of the present invention is a motor including a stator, a rotor, and a case. The stator has a stator core and a winding. The rotor has a first rotor core made of metal, a second rotor core made of metal, and a field magnet. Each of the first and second rotor cores having a plurality of claw-shaped magnetic poles arranged in a peripheral direction. The field magnet is sandwiched between the first rotor core and the second rotor core in an axial direction. The field magnet is configured so as to cause the claw-shaped magnetic pole of the first rotor core and the claw-shaped magnetic pole of the second rotor core to function as magnetic poles different from each other in the sandwiched state. The case is cylindrical with a bottom. The case has a yoke housing which is a magnetic body and a lid portion closing an opening portion of the yoke housing. The yoke housing accommodates the stator and the rotor. In the axial direction, the first rotor core is located closer to the lid portion than the second rotor core. The second rotor core is located closer to a bottom portion of the yoke housing than the first rotor core. The second rotor core is configured so as to be magnetically saturated less easily than the first rotor core.

A third aspect of the present invention is a motor including a stator, a rotor, and a case. The stator has a stator core having a plurality of teeth and a winding wound around the plurality of teeth. Each of the teeth extends in a radial direction. The rotor has a first rotor core, a second rotor core, and a field magnet. Each of the first and second rotor cores has a substantially disc-shaped core base and a plurality of claw-shaped magnetic poles provided at equal intervals on an outer peripheral portion of the core base. Each of the claw-shaped magnetic poles protrudes outside in a radial direction and extends in an axial direction. The core bases are opposed to each other and their claw-shaped magnetic poles are alternately arranged in the peripheral direction. The field magnet is arranged in the axial direction between the core bases. The field magnet is magnetized in the axial direction so as to cause the claw-shaped magnetic pole of the first rotor core to function as a first magnetic pole and the claw-shaped magnetic pole of the second rotor core to function as a second magnetic pole. The case is cylindrical with a bottom. The case has a yoke housing which is a magnetic body and a lid portion closing an opening portion of the yoke housing. The case accommodates the stator and the rotor. The yoke housing has a bottom portion. The bottom portion is configured such that a part of the bottom portion has a sectional area different from the sectional area of the other portions in a section along the axial direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a motor according to a first embodiment of the present invention;

FIG. 2 is a plane view of the motor in FIG. 1;

FIG. 3 is a perspective view of a rotor in FIG. 2;

FIG. 4 is a sectional view along line 4-4 in FIG. 3;

FIG. 5 is an exploded perspective view of the rotor in FIG. 3;

FIG. 6 is a plan view of a rectifying magnet and an annular magnet in FIG. 5;

FIG. 7 is a plan view of the rectifying magnet and the annular magnet in another example of the first embodiment;

FIG. 8 is a plan view of the rectifying magnet and the annular magnet in another example of the first embodiment;

FIG. 9 is a plan view of the rectifying magnet and the annular magnet in another example of the first embodiment;

FIG. 10 is a sectional view of a motor according to a second embodiment of the present invention;

FIG. 11 is a sectional view of the rotor in FIG. 10;

FIG. 12 is a perspective view of a rectifying magnet in FIG. 11;

FIG. 13 is an expansion view illustrating the rectifying magnet in FIG. 12 in a planar state;

FIG. 14 is a sectional view of a motor of a third embodiment of the present invention;

FIG. 15 is a plan view of the motor in FIG. 14;

FIG. 16 is a perspective view of a rotor in FIG. 15;

FIG. 17 is a sectional view schematically illustrating a configuration of a vicinity of the rotor in FIG. 14;

FIG. 18 is a graph for explaining a detent torque of the rotor in FIG. 14;

FIG. 19 is a sectional view of a brushless motor according to a fourth embodiment of the present invention;

FIG. 20 is a plan view of the brushless motor in FIG. 19;

FIG. 21 is a perspective view of a rotor in FIG. 20;

FIG. 22 is a sectional view of the rotor in FIG. 21; and

FIG. 23 is a sectional view of a brushless motor in another example of the fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

A first embodiment of a motor will be described below.

As illustrated in FIG. 1, a case 12 of a brushless motor 11 serving as a motor has a yoke housing 13 formed having a substantially cylindrical shape with a bottom and an end plate 14 serving as a lid portion for closing an opening of a front side (left side in FIG. 1) of this yoke housing 13. The yoke housing 13 is constituted by a magnetic body (iron, for example), for example. The end plate 14 is constituted by a non-magnetic body (a resin material, for example), for example.

As illustrated in FIG. 1, a stator 16 is fixed to an inner peripheral surface of the yoke housing 13. The stator 16 is provided with a stator core 17 having a plurality of teeth 17 a extending to an inside in the radial direction and a winding 20 wound around the teeth 17 a of the stator core 17 through an insulator 19. The stator 16 generates a rotating magnetic field when a driving current is supplied to the winding 20 from an external control circuit S.

As illustrated in FIG. 2, the stator core 17 has twelve teeth 17 a. Therefore, the number of slots 17 b formed between the teeth 17 a is also twelve.

As illustrated in FIG. 2, each of the teeth 17 a is provided with a winding portion 18 a and a protruding portion 18 b protruding to both sides in a peripheral direction from an end portion inside in the radial direction of the winding portion 18 a. In the winding portion 18 a, U-phase, V-phase, and W-phase windings 20 are wound in a concentrated winding.

As illustrated in FIG. 1, a rotor 21 of the brushless motor 11 has a rotating shaft 22 and is arranged inside the stator 16. The rotating shaft 22 is a non-magnetic body metal shaft and is supported rotatably by bearings 23 and 24 supported by a bearing holding portion 13 b of a bottom portion 13 a of the yoke housing 13 and a bearing holding portion 14 a of an end plate 14.

As illustrated in FIGS. 3 to 5, the rotor 21 includes first and second rotor cores 30 and 40, an annular magnet 50 serving as a field magnet interposed between each of the rotor cores 30 and 40 in the axial direction (direction along an axis L), and a rectification magnet 52. The first and second rotor cores 30 and 40 are fixed to the rotating shaft 22 while an axial interval defined therebetween is held by press-fit of the rotating shaft 22.

As illustrated in FIGS. 3 to 5, the first rotor core 30 has a substantially disc-shaped core base 31, and a through hole 31 a is formed at a substantially center position thereof. From an outer peripheral portion on one side in the axial direction of the through hole 31 a, a substantially cylindrical boss portion 31 b is protruded. In the first embodiment, the through hole 31 a and the boss portion 31 b are formed at the same time by burring work. An outer diameter of the boss portion 31 b is smaller than an outer diameter of the bearing 23 rotatably supporting one side of the rotating shaft 22, that is, an inner diameter of the bearing holding portion 13 b for accommodating and fixing the bearing 23 provided on the yoke housing 13.

As illustrated in FIG. 1, the rotating shaft 22 is press-fitted into the through hole 31 a (boss portion 31 b), and the core base 31 is pressed and fixed to the rotating shaft 22. At this time, by means of formation of the boss portion 31 b, the core base 31 is firmly pressed and fixed to the rotating shaft 22. When the core base 31 is pressed and fixed to the rotating shaft 22, the boss portion 31 b is spaced away in the axial direction from the bearing 23 accommodated in and fixed to the bearing holding portion 13 b.

On an outer peripheral surface 31 c of the core base 31 of the first rotor core 30, a plurality of (four in the first embodiment) claw-shaped magnetic poles 32 are formed at equal intervals. Each of the claw-shaped magnetic poles 32 protrudes to an outside in the radial direction and also extends in the axial direction. In more detail, the claw-shaped magnetic pole 32 has a protruding portion 33 protruding to the outside in the radial direction from the outer peripheral portion of the core base 31 and a claw portion 34 provided at a tip end of the protruding portion 33 and extending in the axial direction.

Both end surfaces 32 a and 32 b in the peripheral direction of the claw-shaped magnetic pole 32 are flat surfaces extending in the radial direction (not inclined with respect to the radial direction when seen from the axial direction). An angle of each of the claw-shaped magnetic poles 32 in the peripheral direction, that is, an angle between the both end surfaces 32 a and 32 b in the peripheral direction is set smaller than an angle of a gap between the claw-shaped magnetic poles adjacent to each other in the peripheral direction.

Moreover, an outer surface f1 in the radial direction of the claw portion 34 has an arc-shaped section around the axis L of the rotating shaft 22 in a direction orthogonal to the axis L. The outer surface f1 in the radial direction has two grooves, that is, a first auxiliary groove 35 and a second auxiliary groove 36. The first auxiliary groove 35 and the second auxiliary groove 36 are formed at respective positions shifted in the peripheral direction only by the same angle to both sides from the center in the peripheral direction of the outer surface f1. Each of the first auxiliary groove 35 and the second auxiliary groove 36 has a substantially U-shaped section in the direction orthogonal to the axis L.

Moreover, in the core base 31 of the first rotor core 30, four attachment holes 37 are provided at equal angular intervals on a circle around the axis L. Each of the attachment holes 37 penetrates the core base 31.

As illustrated in FIGS. 3 to 5, the second rotor core 40 is constituted to have the same material and shape as those of the first rotor core 30.

The second rotor core 40 has a substantially disc-shaped core base 41, and a through hole 41 a is formed at a substantially center position of the core base 41. From an outer peripheral portion on the other side in the axial direction of the through hole 41 a, a substantially cylindrical boss portion 41 b is protruded. In the first embodiment, the through hole 41 a and the boss portion 41 b are formed at the same time by burring work. An outer diameter of the boss portion 41 b is smaller than an outer diameter of the bearing 24 rotatably supporting one side of the rotating shaft 22, that is, an inner diameter of the bearing holding portion 14 a for accommodating and fixing the bearing 24 provided on the end plate 14.

The rotating shaft 22 is press-fitted into the through hole 41 a (boss portion 41 b), and the core base 41 is pressed and fixed to the rotating shaft 22. At this time, by means of formation of the boss portion 41 b, the core base 41 is firmly pressed and fixed to the rotating shaft 22. When the core base 41 is pressed and fixed to the rotating shaft 22, the boss portion 41 b is spaced away in the axial direction from the bearing 24 accommodated in and fixed to the bearing holding portion 14 b.

On an outer peripheral surface 41 c of the core base 41 of the second rotor core 40, a plurality of (four in the first embodiment) claw-shaped magnetic poles 42 are formed at equal intervals. Each of the claw-shaped magnetic poles 42 protrudes to an outside in the radial direction and also extends in the axial direction. In more detail, the claw-shaped magnetic pole 42 has a protruding portion 43 protruding to the outside in the radial direction from the outer peripheral portion of the core base 41 and a claw portion 44 provided at a tip end of the protruding portion 43 and extending in the axial direction.

Both end surfaces 42 a and 42 b in the peripheral direction of the claw-shaped magnetic pole 42 are flat surfaces extending in the radial direction (not inclined with respect to the radial direction when seen from the axial direction). An angle of each of the claw-shaped magnetic poles 42 in the peripheral direction, that is, an angle between the both end surfaces 42 a and 42 b in the peripheral direction is set smaller than an angle of a gap between the claw-shaped magnetic poles adjacent to each other in the peripheral direction.

Moreover, an outer surface f2 in the radial direction of the claw portion 44 has an arc-shaped section around the axis L of the rotating shaft 22 in a direction orthogonal to the axis L. The outer surface f2 in the radial direction has two grooves, that is, a first auxiliary groove 45 and a second auxiliary groove 46. The first auxiliary groove 45 and the second auxiliary groove 46 are formed at respective positions shifted in the peripheral direction only by the same angle to both sides from the center in the peripheral direction of the outer surface f2. Each of the first auxiliary groove 45 and the second auxiliary groove 46 has a substantially U-shaped section in the direction orthogonal to the axis L.

Moreover, the core base 41 of the second rotor core 40 has four attachment holes 47 located at equal angular intervals on a circle around the axis L. Each of the attachment holes 47 penetrates the core base 41. Into this attachment hole 47, a projection 61 c of a support plate 61 holding a sensor magnet 62 is inserted. The attachment hole 47 is provided in order to attach the support plate 61 to the rotor 21.

Then, the rotating shaft 22 is press-fitted into the through holes 31 a and 41 a of the rotor cores 30 and 40. The rotor cores 30 and 40 are fixed to the rotating shaft 22 so that a distance between the outer surfaces in the axial direction of the core bases 31 and 41 (distance between opposed side surfaces) becomes a distance set in advance. At this time, the second rotor core 40 is assembled to the rotor core 30 so that the annular magnet 50 is arranged (sandwiched) between the core base 41 and the core base 31 in the axial direction. At this time, each of the claw-shaped magnetic poles 42 is arranged between the claw-shaped magnetic poles 32 of the first rotor core 30 adjacent to each other in the peripheral direction.

As illustrated in FIGS. 4 to 6, the annular magnet 50 is a disc-shaped permanent magnet, and a through hole 50 a is formed in a center part thereof. A cylindrical sleeve 51 is inserted into the through hole 50 a of the annular magnet 50. The sleeve 51 is made of a non-magnetic body and is formed by stainless similarly to the rotating shaft 22 in the first embodiment. An outer peripheral surface of the sleeve 51 and an inner peripheral surface of the through hole 50 a of the annular magnet 50 are bonded by an adhesive made of a hardening resin not permeable to a magnetic flux.

An outer diameter of the annular magnet 50 matches the outer diameters of the core base 31 of the first rotor core 30 and the core base 41 of the second rotor core 40. Therefore, an outer peripheral surface 50 b of the annular magnet 50 becomes flush with the outer peripheral surface 31 c of the core base 31 of the first rotor core 30 and the outer peripheral surface 41 c of the core base 41 of the second rotor core 40.

The annular magnet 50 is magnetized in the axial direction so that the claw-shaped magnetic pole 32 of the first rotor core 30 functions as the N-pole, while the claw-shaped magnetic pole 42 of the second rotor core 40 functions as the S-pole. That is, the rotor 21 of the first embodiment is a rotor with a so-called Lundell structure using the annular magnet 50 as a field magnet. The rotor 21 is constituted such that the four claw-shaped magnetic poles 32 which are the N-poles and the four claw-shaped magnetic poles 42 which are the S-poles are arranged alternately in the peripheral direction. The number of poles of the rotor 21 is eight (the number of pole pairs is four). That is, the brushless motor 11 of the first embodiment is constituted such that the number of poles of the rotor 21 is “8” and the number of teeth 17 a of the stator 16 is “12”.

As illustrated in FIGS. 4 to 6, the rectification magnet 52 includes a first back-surface magnet portion 53, a second back-surface magnet portion 54, and an inter-pole magnet portion 55. The rectification magnet 52 is a polar anisotropic magnet magnetized so that each of the back-surface magnet portions 53 and 54 and the inter-pole magnet portion 55 suppresses leakage flux. The annular magnet 50 and the rectification magnet 52 are constituted by different materials. Specifically, the annular magnet 50 is an anisotropic sintered magnet, for example, and is constituted by a ferrite magnet, a samarium cobalt (SmCo) magnet, a neodymium magnet, an alnico (AlNiCo) magnet and the like, for example. The rectification magnet 52 is a bond magnet (plastic magnet, rubber magnet and the like), for example, and can be constituted by a ferrite magnet, a samarium-iron-nitrogen (SmFeN) magnet, a samarium cobalt (SmCo) magnet, a neodymium magnet, an alnico (AlNiCo) magnet and the like, for example.

The first back-surface magnet portion 53 is arranged between a back surface 34 a of the claw-shaped magnetic pole 32 (claw portion 34) of the first rotor core 30 and the outer peripheral surface 41 c of the core base 41 of the second rotor core 40. The back-surface magnet portion 53 is magnetized mainly in the radial direction component such that a portion in contact with the back surface 34 a (inner peripheral surface) of the claw portion 34 becomes the S-pole which is the same pole as its claw portion 34 and a portion in contact with the outer peripheral surface 41 c of the core base 41 of the second rotor core 40 becomes the N-pole which is the same pole as its core base 41.

The second back-surface magnet portion 54 is arranged between a back surface 44 a of the claw-shaped magnetic pole 42 (claw portion 44) of the second rotor core 40 and an outer peripheral surface 31 c of the core base 31 of the first rotor core 30. The back-surface magnet portion 54 is magnetized mainly in the radial direction component such that a portion in contact with the back surface 44 a (inner peripheral surface) of the claw portion 44 becomes the N-pole which is the same pole as its claw portion 44 and a portion in contact with the outer peripheral surface 31 c of the core base 31 of the first rotor core 30 becomes the S-pole which is the same pole as its core base 31.

As illustrated in FIG. 6, as a volume of the back-surface magnet portion 53 is different from a volume of the back-surface magnet portion 54, the back-surface magnet portion 53 has a thickness T1 in the radial direction larger than a thickness T2 in the radial direction of the back-surface magnet portion 54 (T1>T2). More specifically, an outer surface 53 a in the radial direction of the back-surface magnet portion 53 and an outer surface 54 a in the radial direction of the back-surface magnet portion 54 are located on a circle around a center in the radial direction (axis L) of the rectification magnet 52. Then, a distance L1 between an inner surface 53 b in the radial direction of the back-surface magnet portion 53 and the center in the radial direction (axis L) is shorter than a distance L2 between an inner surface 54 b in the radial direction of the back-surface magnet portion 54 and the center in the radial direction (axis L). Thus, when the annular magnet 50 is arranged inside in the radial direction of the rectification magnet 52, for example, a gap G is provided between the inner surface 54 b in the radial direction of the back-surface magnet portion 54 and the outer peripheral surface 50 b of the annular magnet 50. Moreover, since an outer diameter of the annular magnet 50 and an outer diameter of each of the core bases 31 and 41 are substantially the same, as illustrated in FIGS. 2 and 3, the gap G is provided also between the inner surface 54 b in the radial direction of the back-surface magnet portion 54 and the outer peripheral surface 31 c of the core base 31.

An inter-pole magnet portion 55 is arranged between the first claw-shaped magnetic pole 32 and the second claw-shaped magnetic pole 42 in the peripheral direction. The inter-pole magnet portion 55 is magnetized mainly in the peripheral direction component so that a portion closer to the first claw-shaped magnetic pole 32 in the peripheral direction is the N-pole and a portion closer to the second claw-shaped magnetic pole 42 is the S-pole.

As illustrated in FIG. 1, on an end surface closer to the end plate 14 of the rotor 21 (an end surface of the core base 41 of the second rotor core 40), the support plate 61 holding the sensor magnet 62 is fixed. The support plate 61 is formed of a non-magnetic body.

As illustrated in FIG. 1, the support plate 61 has a disc-shaped base portion 61 a. At a center part of the base portion 61 a, a through hole through which the rotating shaft 22 is penetrated is formed. A surface of the base portion 61 a closer to the second rotor core 40 has four projections 61 c (only one of them is shown in FIG. 1) arranged at equal angular intervals. The projections 61 c are formed by pressing and protrude from the surface of the base portion 61 a. The projections 61 c are fitted in the attachment holes 47 formed in the core base 41 of the second rotor core 40, respectively. At this time, the base portion 61 a is brought into contact with the end surface of the core base 41 of the second rotor core 40 in the axial direction and with a part of the rectification magnet 52 (end surface in the axial direction of the back-surface magnet portion 53 and the end surface in the axial direction of the inter-pole magnet portion 55) in the axial direction.

On an outer peripheral edge portion of the base portion 61 a, a cylindrical wall 61 b extending toward a side opposite to the rotor 21 in the axial direction (extending toward the end plate 14) is provided. An outer diameter of the cylindrical wall 61 b is substantially equal to the outer diameter of the rotor 21.

As illustrated in FIG. 1, on an inner peripheral surface of the cylindrical wall 61 b, the ring-shaped sensor magnet 62 is provided. An outer surface in the radial direction of the sensor magnet 62 is fixed to the inner peripheral surface of the cylindrical wall 61 b by an adhesive. At this time, the sensor magnet 62 is fixed to the support plate 61 so that an axis of the sensor magnet 62 matches the axis L of the rotating shaft 22. As described above, the sensor magnet 62 is constituted capable of integral rotation with the rotor 21.

On the end plate 14, a magnetic sensor 63 such as a hall IC or the like faced with the sensor magnet 62 in the axial direction is provided. This magnetic sensor 63 outputs an H-level detection signal and an L-level detection signal to the control circuit S when it senses a magnetic field of the N-pole and a magnetic field of the S-pole of the sensor magnet 62, respectively.

Subsequently, an action of the brushless motor 11 constituted as above will be described.

When a three-phase driving current is supplied to the winding 20 from the control circuit S, a rotating magnetic field is generated in the stator 16, and the rotor 21 is rotated/driven. At this time, rotation of the sensor magnet 62 faced with the magnetic sensor 63 causes a level of the detection signal outputted from the magnetic sensor 63 to switch in accordance with a rotation angle (position) of the rotor 21, and the three-phase driving current switched at an optimal timing is supplied from the control circuit S to the winding 20 on the basis of the detection signal. As a result, the rotating magnetic field is favorably generated, and the rotor 21 is continuously rotated/driven favorably.

Here, the core base 31 of the first rotor core 30 is closer to the bottom portion 13 a of the yoke housing 13 in the axial direction than the core base 41 of the second rotor core 40. As a result, a leakage flux can occur easily between the core base 31 of the first rotor core 30 and the yoke housing 13 (bottom portion 13 a). In the first embodiment, the thickness T1 of the back-surface magnet portion 53 provided on the back surface of the claw-shaped magnetic pole 32 (claw portion 34) of the first rotor core 30 is larger than the thickness T2 of the back-surface magnet portion 54 provided on the back surface of the claw-shaped magnetic pole 42 (claw portion 44) of the second rotor core 40. Thus, a magnetic flux amount of the back-surface magnet portion 53 is larger than the magnetic flux amount of the back-surface magnet portion 54. As a result, even if a leakage flux occurs between the core base 31 of the first rotor core 30 and the bottom portion 13 a, a magnetic balance can be made favorable.

Subsequently, advantages of the first embodiment will be described.

(1) The back-surface magnet portion 53 provided on the back surface of the claw-shaped magnetic pole 32 of the first rotor core 30 has a thickness larger than the thickness of the back-surface magnet portion 54 provided on the back surface of the claw-shaped magnetic pole 42 of the second rotor core 40. As a result, since the magnetic flux amount of the back-surface magnet portion 53 becomes relatively large, even if a leakage flux occurs, the magnetic balance can be made favorable.

(2) The back-surface magnet portions 53 and 54 are integrally formed with the inter-pole magnet portion 55. Thus, an increase in the number of components can be suppressed.

Second Embodiment

Subsequently, a second embodiment of a motor will be described.

The motor of the second embodiment is different from the first embodiment in a configuration of a rotor, and the stator has the same configuration. Thus, the rotor will be mainly described, and the same reference numerals are given to the other configurations, and a part of or the whole of the explanation will be omitted.

As illustrated in FIGS. 10 and 11, the rotor 21 of the brushless motor 11 has the rotating shaft 22 and is arranged inside the stator 16. The rotating shaft 22 is a non-magnetic body metal shaft and is supported rotatably by the bearings 23 and 24 supported by the bearing holding portion 13 b of the bottom portion 13 a of the yoke housing 13 and the bearing holding portion 14 a of the end plate 14.

As illustrated in FIG. 11, the rotor 21 includes the first and second rotor cores 30 and 40, the annular magnet 50 serving as a field magnet interposed between the rotor cores 30 and 40 in the axial direction, and the rectification magnet 100. The first and second rotor cores 30 and 40 are fixed to the rotating shaft 22 while an axial interval defined therebetween is held by press-fit of the rotating shaft 22.

On the outer peripheral surface 31 c of the core base 31 of the first rotor core 30, a plurality of (four in the second embodiment) claw-shaped magnetic poles 32 are formed at equal intervals. Each of the claw-shaped magnetic poles 32 protrudes to an outside in the radial direction and also extends in the axial direction. In more detail, the claw-shaped magnetic pole 32 has the protruding portion 33 protruding to the outside in the radial direction from the outer peripheral portion of the core base 31 and the claw portion 34 provided at a tip end of the protruding portion 33 and extending in the axial direction.

On the outer peripheral surface 41 c of the core base 41 of the second rotor core 40, a plurality of (four in the second embodiment) claw-shaped magnetic poles 42 are formed at equal intervals. Each of the claw-shaped magnetic poles 42 protrudes to an outside in the radial direction and also extends in the axial direction. In more detail, the claw-shaped magnetic pole 42 has the protruding portion 43 protruding to the outside in the radial direction from the outer peripheral portion of the core base 41 and the claw portion 44 provided at a tip end of the protruding portion 43 and extending in the axial direction.

Moreover, the claw-shaped magnetic pole 32 of the first rotor core 30 (claw portion 34) has an axial length L3 longer than an axial length L4 of the claw-shaped magnetic pole 42 of the second rotor core 40 (claw portion 44). Thus, in a state in which the core bases 31 and 41 of the rotor cores 30 and 40 and the annular magnet 50 are assembled, a tip end surface 44 b of the claw-shaped magnetic pole 42 of the second rotor core 40 (claw portion 44) is located on a side opposite to the yoke housing 13 in the axial direction from an axial end surface 31 d of the core base 31 of the first rotor core 30. That is, the tip end surface 44 b is located at a position spaced away from the yoke housing 13 from the end surface 31 d. As a result, a separation distance D1 in the axial direction between the tip end surface 44 b of the claw-shaped magnetic pole 42 of the second rotor core 40 (claw portion 44) and the yoke housing 13 becomes longer than a separation distance D2 in the axial direction between the axial end surface 31 d of the core base 31 of the first rotor core 30 and the yoke housing 13.

Then, the rotating shaft 22 is press-fitted into the through holes 31 a and 41 a of the rotor cores 30 and 40. The rotor cores 30 and 40 are press-fitted and fixed to the rotating shaft 22 so that an interval between the axial outer surfaces of the core bases 31 and 41 (distance between opposing side surfaces) becomes a distance set in advance. At this time, the second rotor core 40 is assembled to the rotor core 30 so that the annular magnet 50 is arranged (sandwiched) in the axial direction between the core base 41 and the core base 31. At this time, each of the claw-shaped magnetic poles 42 is arranged between the claw-shaped magnetic poles 32 adjacent to each other in the peripheral direction of the other first rotor core 30.

As illustrated in FIG. 11, the annular magnet 50 is a disc-shaped permanent magnet, and the through hole 50 a is formed in the center part thereof. The cylindrical sleeve 51 is inserted into the through hole 50 a of the annular magnet 50. The sleeve 51 is made of a non-magnetic body and is formed by stainless similarly to the rotating shaft 22 in the second embodiment. The outer peripheral surface of the sleeve 51 and the inner peripheral surface of the through hole 50 a of the annular magnet 50 are fixed by an adhesive made of a hardening resin not permeable to a magnetic flux. The annular magnet 50 is magnetized in the axial direction so that the claw-shaped magnetic pole 32 of the first rotor core 30 functions as the N-pole, while the claw-shaped magnetic pole 42 of the second rotor core 40 functions as the S-pole. That is, the rotor 21 of the second embodiment is a rotor with a so-called Lundell structure using the annular magnet 50 as a field magnet.

As illustrated in FIGS. 12 and 13, a rectification magnet 100 includes a first back-surface magnet portion 101, a second back-surface magnet portion 102, and the inter-pole magnet portion 55. The rectification magnet 100 is a polar anisotropic magnet magnetized so that each of the back-surface magnet portions 101 and 102 and the inter-pole magnet portion 55 suppresses leakage flux. The annular magnet 50 and the rectification magnet 100 are constituted by a material similar to that in the above-described first embodiment.

The first back-surface magnet portion 101 is arranged between a back surface 34 a of the claw-shaped magnetic pole 32 (claw portion 34) of the first rotor core 30 and the outer peripheral surface 41 c of the core base 41 of the second rotor core 40. The back-surface magnet portion 101 is magnetized mainly in the radial direction component such that a portion in contact with the back surface 34 a (inner peripheral surface) of the claw portion 34 becomes the S-pole which is the same pole as its claw portion 34 and a portion in contact with the outer peripheral surface 41 c of the core base 41 of the second rotor core 40 becomes the N-pole which is the same pole as its core base 41.

The second back-surface magnet portion 102 is arranged between a back surface 44 a of the claw-shaped magnetic pole 42 (claw portion 44) of the second rotor core 40 and an outer peripheral surface 31 c of the core base 31 of the first rotor core 30. The back-surface magnet portion 102 is magnetized mainly in the radial direction component such that a portion in contact with the back surface 44 a (inner peripheral surface) of the claw portion 44 becomes the N-pole which is the same pole as its claw portion 44 and a portion in contact with the outer peripheral surface 31 c of the core base 31 of the first rotor core 30 becomes the S-pole which is the same pole as its core base 31.

As illustrated in FIG. 13, the back-surface magnet portion 102 has an axial length L6 shorter than the axial length L5 of the back-surface magnet portion 101 (L5>L6) so as to differentiate a volume of the back-surface magnet portion 102 from a volume of the back-surface magnet portion 101.

The inter-pole magnet portion 55 is arranged in the peripheral direction between the first claw-shaped magnetic pole 32 and the second claw-shaped magnetic pole 42. The inter-pole magnet portion 55 is magnetized mainly in the peripheral direction component so that a portion closer to the first claw-shaped magnetic pole 32 in the peripheral direction is the N-pole and a portion closer to the second claw-shaped magnetic pole 42 is the S-pole.

Subsequently, an action of the brushless motor 11 configured as above will be described.

When a three-phase driving current is supplied to the winding 20 from the control circuit S, a rotating magnetic field is generated in the stator 16, and the rotor 21 is rotated/driven. At this time, rotation of the sensor magnet 62 faced with the magnetic sensor 63 causes a level of the detection signal outputted from the magnetic sensor 63 to switch in accordance with a rotation angle (position) of the rotor 21, and the three-phase driving current switched at an optimal timing is supplied from the control circuit S to the winding 20 on the basis of the detection signal. As a result, the rotating magnetic field is favorably generated, and the rotor 21 is continuously rotated/driven favorably.

Here, in the second embodiment, the back-surface magnet portion 102 on the back surface of the claw-shaped magnetic pole 42 of the second rotor core 40 has an axial length L6 shorter than an axial length L5 of the back-surface magnet portion 101 on the back surface of the claw-shaped magnetic pole 32 of the first rotor core 30. Thus, a distance between the claw-shaped magnetic pole 42 of the second rotor core 40 and the yoke housing 13 becomes longer, and the leakage flux can be suppressed. As a result, imbalance of the magnetic flux between the claw-shaped magnetic pole 32 and the claw-shaped magnetic pole 42 is suppressed.

Moreover, the claw-shaped magnetic pole 32 of the first rotor core 30 has the axial length longer than the axial length of the claw-shaped magnetic pole 42 of the second rotor core 40. As a result, magnetic resistance between the claw-shaped magnetic pole 32 of the first rotor core 30 and the stator 16 becomes lower than the magnetic resistance between the claw-shaped magnetic pole 42 of the second rotor core 40 and the stator 16. Here, the magnetic flux can easily leak to the yoke housing 13 from the core base 31 of the first rotor core 30 closer to the yoke housing 13 in the axial direction. However, as described above, the claw-shaped magnetic pole 32 of the first rotor core 30 has the axial length longer than the axial length of the claw-shaped magnetic pole 42 of the second rotor core 40. As a result, the magnetic resistance of the first rotor core 30 becomes relatively low and thus, leakage of the magnetic flux to the yoke housing 13 is suppressed. Moreover, regarding the second rotor core 40 relatively farther from the yoke housing 13, the leakage flux to the yoke housing 13 is less. Thus, assuming that the axial lengths of the claw-shaped magnetic pole 32 of the first rotor core 30 and the claw-shaped magnetic pole 42 of the second rotor core 40 are the same, the magnetic flux amount between the claw-shaped magnetic pole 42 of the second rotor core 40 and the stator 16 tends to become larger than the magnetic flux amount between the claw-shaped magnetic pole 32 of the first rotor core 30 and the stator 16. That is, the magnetic flux becomes imbalanced between the claw-shaped magnetic pole 32 and the claw-shaped magnetic pole 42. Thus, by making the axial length of the claw-shaped magnetic pole 42 of the second rotor core 40 shorter than the axial length of the claw-shaped magnetic pole 32 of the first rotor core 30, the magnetic resistance between the claw-shaped magnetic pole 42 of the second rotor core 40 and the stator 16 becomes high, and the magnetic flux imbalance between the claw-shaped magnetic pole 32 and the claw-shaped magnetic pole 42 can be suppressed.

Subsequently, the advantages of the second embodiment will be described.

(3) The leakage flux can easily occur between the back-surface magnet portion 102 and the yoke housing 13. The back-surface magnet portion 102 provided on the back surface of the claw-shaped magnetic pole 42 of the second rotor core 40 has the axial length shorter than the back-surface magnet portion 101 provided on the back surface of the claw-shaped magnetic pole 32 of the first rotor core 30. As a result, the interval between the yoke housing 13 and the back-surface magnet portion 102 is spaced away, whereby occurrence of the leakage flux can be suppressed, and the magnetic balance between the claw-shaped magnetic pole 32 and the claw-shaped magnetic pole 42 can be made favorable.

(4) The magnetic flux can easily leak to the yoke housing 13 from the core base 31 of the first rotor core 30 closer in the axial direction to the yoke housing 13 which is a magnetic body. However, by making the claw-shaped magnetic pole 32 of the first rotor core 30 longer, the magnetic resistance in the radial direction between the claw-shaped magnetic pole 32 and the stator 16 becomes small. Thus, the leakage flux to the yoke housing 13 can be reduced. As a result, output characteristics can be improved. Moreover, regarding the second rotor core 40 which is relatively farther from the yoke housing 13, the leakage flux to the yoke housing 13 is less. Thus, if the claw-shaped magnetic pole 32 of the first rotor core 30 has the same axial length as the axial length of the claw-shaped magnetic pole 42 of the second rotor core 40, the magnetic flux amount between the claw-shaped magnetic pole 42 of the second rotor core 40 and the stator 16 tends to become larger than the magnetic flux amount between the claw-shaped magnetic pole 32 of the first rotor core 30 and the stator 16. That is, the magnetic flux imbalance occurs between the claw-shaped magnetic pole 32 and the claw-shaped magnetic pole 42. Thus, since the claw-shaped magnetic pole 42 of the second rotor core 40 has the axial length shorter than the axial length of the claw-shaped magnetic pole 32 of the first rotor core 30, the magnetic resistance between the claw-shaped magnetic pole 42 of the second rotor core 40 and the stator 16 becomes high. As a result, the magnetic flux imbalance between the claw-shaped magnetic pole 32 and the claw-shaped magnetic pole 42 is further suppressed, and the magnetic balance between the claw-shaped magnetic pole 32 and the claw-shaped magnetic pole 42 can be made favorable.

(5) The distance in the axial direction between the tip end surface 44 b of the claw-shaped magnetic pole 42 of the second rotor core 40 and the yoke housing 13 can be ensured. Thus, the leakage flux from the tip end surface 44 b of the claw-shaped magnetic pole 42 to the yoke housing 13 can be suppressed, and an interlinkage flux amount to the stator 16 can be increased.

(6) The back-surface magnet portions 101 and 102 are integrally formed with the inter-pole magnet portion 55. Thus, an increase in the number of components can be suppressed.

The above-described first and second embodiments can be changed as follows.

In the first and second embodiments, the rectification magnets 52 and 100 do not have to be constituted by a polar anisotropic magnet.

As illustrated in FIG. 7, the rectification magnet 52 may be magnetized such that a magnetic orientation direction of the inter-pole magnet portion 55 is a peripheral direction, and the magnetic orientation directions of the back-surface magnet portions 53 and 54 are radial directions.

In the first and second embodiments, the outer peripheral surface 50 b of the annular magnet 50 does not have to be circular.

For example, a configuration illustrated in FIG. 8 may be employed. A projection portion 50 c is provided on the outer peripheral surface 50 b of the annular magnet 50. The projection portion 50 c abuts against the relatively thin back-surface magnet portion 54 and fills the gap G. Such configuration can increase a main magnetic flux component.

Moreover, a configuration illustrated in FIG. 9 may be employed. The back-surface magnet portion 53 is the thickest at a substantial center in the peripheral direction and becomes gradually thinner toward the outer side in the peripheral direction. The back-surface magnet portion 54 is the thinnest at the substantial center in the peripheral direction and becomes gradually thicker toward the outer side in the peripheral direction. In this case, it is preferable that the inter-pole magnet portion 55 has a thickness gradually changing so as to continue to the adjacent back-surface magnet portions 53 and 54. Moreover, as indicated by a solid line in FIG. 9, the annular magnet 50 may have a shape following the back-surface magnet portions 53 and 54 and the inter-pole magnet portion 55. Moreover, as indicated by a broken line in FIG. 9, the annular magnet 50 may have a circular outer peripheral surface 50 b. In this configuration, the annular magnet 50 is inscribed in a substantial center in the peripheral direction of the back-surface magnet portion 53 which is the thickest portion.

In the first and second embodiments, the inter-pole magnet portion 55 and the back-surface magnet portions 53, 54, 101, and 102 may be separate bodies.

In the first and second embodiments, the inter-pole magnet portion 55 may be omitted.

In the first and second embodiments, the support plate 61 and the sensor magnet 62 are supported by the rotor 21, but it is only necessary that the sensor magnet 62 is configured to be capable of integral rotation with the rotating shaft 22. As an example, the support plate 61 may be fixed (press-fit/fix, for example) to the rotating shaft 22 by giving an interval in the axial direction with respect to the rotor 21 (second rotor core 40).

In the first and second embodiments, the sleeve 51 and the annular magnet 50 are fixed by an adhesive made of a hardening resin not permeable to a magnetic flux. The sleeve 51 and the annular magnet 50 may be fixed by an adhesive permeable to the magnetic flux.

In the first and second embodiments, the sleeve 51 which is a non-magnetic body is made of stainless. It is only necessary that the sleeve 51 is non-magnetic and the sleeve 51 may be made of aluminum or a resin other than stainless.

In the first and second embodiments, the sleeve 51 is interposed between the annular magnet 50 and the rotating shaft 22, but the sleeve 51 may be omitted. In this case, the annular magnet 50 can be directly fixed to the rotating shaft 22.

In the first and second embodiments, the number of poles of the rotor 21 is set to “8”, and the number of the teeth 17 a of the stator 16 is set to “12”. The number of poles of the rotor 21 and the number of teeth 17 a of the stator 16 may be changed. For example, the number of poles of the rotor 21 may be set to “8”, and the number of teeth 17 a of the stator 16 may be set to “12”.

In the first and second embodiments, the stator 16 does not have to be configured such that the winding 20 is wound around the teeth 17 a. For example, the stator core may be configured by combining a first stator core having a plurality of claw-shaped magnetic poles in the peripheral direction and a second stator core having a plurality of claw-shaped magnetic poles in the peripheral direction. In this case, the winding is arranged between the first stator core and the second stator core, and the winding causes the plurality of claw-shaped magnetic poles aligned in the peripheral direction to function as alternately different magnetic poles.

The first and second embodiments and each of the variations may be combined as appropriate.

Third Embodiment

Subsequently, a third embodiment of a motor will be described.

As illustrated in FIG. 14, a motor case 212 of a brushless motor 211 as a motor has a yoke housing 213 formed having a substantially cylindrical shape with a bottom and an end frame 214 as a lid portion for closing an opening of a front side (left side in FIG. 14) in the axial direction of this yoke housing 213. The yoke housing 213 is constituted by a magnetic body (iron, for example), for example. The end frame 214 is constituted by a non-magnetic body (a resin material, for example), for example.

As illustrated in FIGS. 14 and 15, a stator 216 is fixed to an inner peripheral surface of the yoke housing 213. The stator 216 is provided with a stator core 217 having a plurality of teeth 217 a extending to an inside in the radial direction and a winding 220 wound around the teeth 217 a of the stator core 217 through an insulator 219. The stator 216 generates a rotating magnetic field when a driving current is supplied to the winding 220 from the external control circuit S.

As illustrated in FIG. 15, the stator core 217 has twelve teeth 217 a. Therefore, the number of slots 217 b formed between the teeth 217 a is also twelve. Each of the teeth 217 a is provided with a winding portion 218 a and a protruding portion 218 b protruding to both sides in a peripheral direction from an end portion inside in the radial direction of the winding portion 218 a. In the winding portion 218 a, U-phase, V-phase, and W-phase windings 220 are wound in a concentrated winding.

As illustrated in FIG. 14, a rotor 221 of the brushless motor 211 has a rotating shaft 222 and is arranged inside the stator 216. The rotating shaft 222 is a non-magnetic body metal shaft and is supported rotatably by bearings 223 and 224 supported by a bottom portion 213 a of the yoke housing 213 and an end frame 214.

As illustrated in FIGS. 16 and 17, the rotor 221 includes first and second rotor cores 231 and 232, an annular magnet 233 as a field magnet interposed between the first and second rotor cores 231 and 232 in the axial direction (direction along an axis L). The first and second rotor cores 231 and 232 are fixed to the rotating shaft 222 while an axial interval between each is held by press-fit of the rotating shaft 222. Moreover, the rotor 221 includes back-surface auxiliary magnets 234 and 235 and inter-pole magnets 236 and 237.

As illustrated in FIG. 14, in a state in which the rotor 221 is arranged in a motor case 212, the rotor core having a core base 231 a which will be described later arranged closer to the end frame 214 in the axial direction with respect to the annular magnet 233 is made the first rotor core 231, and the rotor core having a core base 232 a arranged closer to the bottom portion 213 a in the axial direction with respect to the annular magnet 233 is made the second rotor core 232.

As illustrated in FIGS. 16 and 17, the first rotor core 231 has a substantially disc-shaped core base 231 a and a plurality of (four in the third embodiment) claw-shaped magnetic poles 231 b formed at equal intervals on an outer peripheral portion of the core base 231 a. Each of the claw-shaped magnetic poles 231 b protrudes to an outside in the radial direction and also extends in the axial direction. In more detail, the claw-shaped magnetic pole 231 b has a protruding portion 231 c protruding to the outside in the radial direction from the outer peripheral portion of the core base 231 a and a claw portion 231 d provided at a tip end of the protruding portion 231 c and extending in the axial direction. The protruding portion 231 c is formed having a fan shape when seen from the axial direction. The claw portion 231 d has a fan-shaped section in a direction orthogonal to the axis L.

The second rotor core 232 has the same shape as the first rotor core 231. The second rotor core 232 has a substantially disc-shaped core base 232 a and a plurality of claw-shaped magnetic poles 232 b formed at equal intervals on an outer peripheral portion of the core base 232 a. Each of the claw-shaped magnetic poles 232 b protrudes to an outside in the radial direction and also extends in the axial direction. In more detail, the claw-shaped magnetic pole 232 b has a protruding portion 232 c protruding to the outside in the radial direction from the outer peripheral portion of the core base 232 a and a claw portion 232 d provided at a tip end of the protruding portion 232 c and extending in the axial direction. The protruding portion 232 c is formed having a fan shape when seen from the axial direction. The claw portion 232 d has a fan-shaped section in a direction orthogonal to the axis L.

Then, the rotating shaft 222 is press-fitted into center holes of the rotor cores 231 and 232. The rotor cores 231 and 232 are fixed to the rotating shaft 222 so that a distance between outer surfaces in the axial direction of the core bases 231 a and 232 a (distance between opposing side surfaces) becomes a distance set in advance. At this time, each of the claw-shaped magnetic poles 232 b of the second rotor core 232 is arranged between each of the claw-shaped magnetic poles 231 b of the first rotor core 231. Moreover, the annular magnet 233 is arranged (sandwiched) between each of the core bases 231 a and 232 a of the first and second rotor cores 231 and 232 in the axial direction.

The annular magnet 233 is a magnet such as a ferrite magnet, a neodymium magnet or the like and is formed annularly having a center hole formed. The annular magnet 233 is magnetized in the axial direction so that the claw-shaped magnetic pole 231 b of the first rotor core 231 functions as a first magnetic pole (N-pole in the third embodiment) and the claw-shaped magnetic pole 232 b of the second rotor core 232 functions as a second magnetic pole (S-pole in the third embodiment). That is, the rotor 221 of the third embodiment is a rotor with a so-called Lundell structure using the annular magnet 233 as a field magnet.

The rotor 221 is configured such that the four claw-shaped magnetic poles 231 b to be the N-poles and the four claw-shaped magnetic poles 232 b to be the S-poles are alternately arranged in the peripheral direction. The number of poles of the rotor 221 is set to eight (the number of pole pairs is four). That is, the brushless motor 211 in the third embodiment is configured such that the number of poles of the rotor 221 is “8”, and the number of teeth 217 a of the stator 216 is “12”. That is, the brushless motor 211 is configured such that the number of poles of the rotor 221 is 2n (where n is a natural number and 4 in the third embodiment), and the number of slots 217 b (slot number) is 3n so that the ratio of the number of poles and the number of slots becomes 2:3.

The first back-surface auxiliary magnet 234 is arranged between a back surface 231 e (surface inside in the radial direction) of each of the claw-shaped magnetic poles 231 b of the first rotor core 231 and an outer peripheral surface 232 f of the core base 232 a of the second rotor core 232. The back-surface auxiliary magnet 234 has a substantially fan-shaped section in the direction orthogonal to the axis L. The back-surface auxiliary magnet 234 is magnetized such that a portion in contact with the back surface 231 e of the claw-shaped magnetic pole 231 b becomes the N-pole which is the same pole as the claw-shaped magnetic pole 231 b and a portion in contact with the outer peripheral surface 232 f of the core base 232 a of the second rotor core 232 becomes the S-pole which is the same pole as its core base 232 a.

Moreover, the second back-surface auxiliary magnet 235 is arranged between a back surface 232 e of each of the claw-shaped magnetic poles 232 b of the second rotor core 232 and an outer peripheral surface 231 f of the core base 231 a of the first rotor core 231. The back-surface auxiliary magnet 235 has a fan-shaped section in the direction orthogonal to the axis L. The back-surface auxiliary magnet 235 is magnetized such that a portion in contact with the back surface 232 e of the claw-shaped magnetic pole 232 b becomes the S-pole and a portion in contact with the outer peripheral surface 231 f of the core base 231 a of the first rotor core 231 becomes the N-pole. As the back-surface auxiliary magnets 234 and 235, a ferrite magnet, for example, can be used.

Moreover, as illustrated in FIGS. 15 and 16, the inter-pole magnets 236 and 237 are arranged between the claw-shaped magnetic pole 231 b and the claw-shaped magnetic pole 232 b in the peripheral direction.

As illustrated in FIGS. 14 and 17, the rotor 221 is configured such that the core base 232 a of the second rotor core 232 is located at a position closer to the yoke housing 213 (bottom portion 213 a) in the axial direction than the core base 231 a of the first rotor core 231.

Here, the first and second rotor cores 231 and 232 are both made of carbon steel. In more detail, for the second rotor core 232 located at the position closer to the bottom portion 213 a of the yoke housing 213, a metal material (low carbon steel) having a content of carbon lower than that of the first rotor core 231 is used. As a result, saturation magnetic flux density (limit point of a flow of a magnetic flux per unit area) of the second rotor core 232 becomes higher than the saturation magnetic flux density of the first rotor core 231. Since the first and second rotor cores 231 and 232 have the same shape, the second rotor core 232 is configured such that the second rotor core 232 is magnetically saturated less easily than the first rotor core 231.

Moreover, as illustrated in FIG. 14, in the rotor 221, a sensor magnet 242 is provided through a substantially disc-shaped magnet fixing member 241. In more detail, the magnet fixing member 241 has a disc portion 241 b in which a boss portion 241 a is formed at a center and a cylinder portion 241 c extending cylindrically from an outer edge of this disc portion 241 b. To the magnet fixing member 241, the annular sensor magnet 242 is fixed so that it abuts against an inner peripheral surface of the cylinder portion 241 c and a surface of the disc portion 241 b. The magnet fixing member 241 is located at a position closer to the first rotor core 231, and the boss portions 241 a is externally fitted with the rotating shaft 222 and fixed to the rotating shaft 222.

Then, in the end frame 214, a hall IC 243 as a magnetic sensor is provided at a position opposite in the axial direction to the sensor magnet 242. The hall IC 243 outputs an H-level detection signal and an L-level detection signal to the control circuit S when it senses magnetic fields of the N-pole and the S-pole on the basis of the sensor magnet 242, respectively.

Subsequently, an action of the brushless motor 211 constituted as above will be described.

When a three-phase driving current is supplied from the control circuit S to the winding 220, a rotating magnetic field is generated in the stator 216, and the rotor 221 is rotated/driven. At this time, by means of rotation of the sensor magnet 242 opposite to the hall IC 243, the level of the detection signal outputted from the hall IC 243 is switched in accordance with a rotation angle (position) of the rotor 221, and the three-phase driving current switched at optimal timing is supplied to the winding 220 from the control circuit S on the basis of the detection signal. As a result, the rotating magnetic field is favorably generated, and the rotor 221 is continuously rotated/driven favorably.

Here, assume a case in which the first and second rotor cores 231 and 232 are made of the same material, for example (that is, a case in which the material with the same saturation magnetic flux density is used). In this case, in the first rotor core 231 farther from the bottom portion 213 a of the yoke housing 213 in the axial direction, there is little leakage flux between that and the yoke housing 213 (bottom portion 213 a). Thus, a detent torque can become higher as indicated by X1 in FIG. 18. In the second rotor core 232 closer to the bottom portion 213 a of the yoke housing 213, leakage flux is generated between that and the bottom portion 213 a of the yoke housing 213. Thus, the detent torque can become lower as indicated by X2 in FIG. 18. Thus, the detent torque becomes imbalanced between the N-pole and the S-pole.

Thus, in the third embodiment, the second rotor core 232 closer to the bottom portion 213 a is constituted by a material (low carbon steel) with higher saturation magnetic flux density than the first rotor core 231. That is, since the saturation magnetic flux density of the first rotor core 231 closer to the end frame 214 and with less occurrence of the leakage flux is lower than the saturation magnetic flux density of the second rotor core 232, magnetic saturation can occur more easily in the first rotor core 231. As a result, a magnetic flux amount acting between the claw-shaped magnetic pole 231 b of the first rotor core 231 and the stator 216 decreases. Thus, the detent torque is reduced in the first rotor core 231 as indicated by Y1 in FIG. 18.

Then, by constituting the second rotor core 232 by a material with high saturation magnetic flux density, the magnetic flux flows more easily in the second rotor core 232. Thus, the leakage flux to the yoke housing 213 is reduced, and the detent torque slightly increases as indicated by Y2 in FIG. 18. As described above, a difference between the detent torque at the first rotor core 231 (N-pole) and the detent torque at the second rotor core 232 (S-pole) decreases, and the balance of the detent torque is improved.

Subsequently, advantages of the third embodiment will be described.

(7) The second rotor core 232 closer to the bottom portion 213 a is formed of a metal material with a carbon content lower than that of the first rotor core 231. As a result, the second rotor core 232 has saturation magnetic flux density higher than the saturation magnetic flux density of the first rotor core 231. As a result, even if the first and second rotor cores 231 and 232 have the same shape as in the third embodiment, the second rotor core 232 is constituted so as to be magnetically saturated less easily than the first rotor core 231. Therefore, the magnetic flux is made to flow more easily in the second rotor core 232 closer to the bottom portion 213 a of the yoke housing 213 and as a result, the leakage flux to the yoke housing 213 can be reduced. Thus, imbalance in the magnetic flux amount between the first rotor core 231 and the second rotor core 232 can be suppressed, and the balance of the detent torque can be made favorable.

The third embodiment may be changed as follows.

In the third embodiment, the second rotor core 232 closer to the bottom portion 213 a is formed of a metal material with a carbon content lower than that of the first rotor core 231. As a result, while the first and second rotor cores 231 and 232 have the same shape, the saturation magnetic flux density of the second rotor core 232 is made higher than the saturation magnetic flux density of the first rotor core 231. The third embodiment is not limited to this configuration. For example, the second rotor core 232 may be formed of a metal material subjected to annealing for magnetic characteristic improvement. By applying annealing to the second rotor core 232, while the first and second rotor cores 231 and 232 are formed having the same shape and of the same material, the saturation magnetic flux density of the second rotor core 232 can be made higher than the saturation magnetic flux density of the first rotor core 231.

Moreover, for example, the first rotor core 231 may be formed of a cold rolled steel plate, while the second rotor core 232 may be formed of a hot rolled steel plate. By means of this configuration, too, while the first and second rotor cores 231 and 232 are formed having the same shape and of the same material, the saturation magnetic flux density of the second rotor core 232 can be made higher than the saturation magnetic flux density of the first rotor core 231.

In the third embodiment, the second rotor core 232 is formed of a material having the saturation magnetic flux density higher than the saturation magnetic flux density of the material of the first rotor core 231. As a result, the second rotor core 232 is constituted so as not to be magnetically saturated easily. The third embodiment is not limited to this configuration. For example, a plate thickness (plate thickness of the core base 232 a) in the axial direction of the second rotor core 232 may be larger than a plate thickness (plate thickness of the core base 231 a) in the axial direction of the first rotor core 231. According to this configuration, while the first and second rotor cores 231 and 232 are formed of the same material, the second rotor core 232 is constituted so as not to be magnetically saturated easily.

Each of the above-described variations and the above-described embodiments may be combined as appropriate.

In the third embodiment, the number of poles of the rotor 221 is set to “8”, and the number of teeth 217 a of the stator 216 is set to “12”. The number of the poles of the rotor 221 or the number of teeth 217 a of the stator 216 may be changed as appropriate.

In the third embodiment, the rotor 221 does not have to have the back-surface auxiliary magnets 234 and 235 and the inter-pole magnets 236 and 237. For example, the rotor 221 may have only the back-surface auxiliary magnet, may have only the inter-pole magnet, or does not have to have the back-surface auxiliary magnets and the inter-pole magnets.

In the third embodiment, the stator 216 does not have to be configured such that the winding 220 is wound around the teeth 217 a. For example, the stator core may be configured by combining a first stator core having a plurality of claw-shaped magnetic poles in the peripheral direction and a second stator core having a plurality of claw-shaped magnetic poles in the peripheral direction. In this case, the winding is arranged between the first stator core and the second stator core, and the winding causes the plurality of claw-shaped magnetic poles aligned in the peripheral direction to function as alternately different magnetic poles.

In the third embodiment, the end frame 214 does not have to be constituted by a resin material. For example, the end frame 214 may be constituted by a non-magnetic body material such as aluminum or stainless steel (SUS) and the like. Moreover, the end frame 214 may be constituted by a magnetic body material as long as the condition that the distance in the axial direction between the end frame 214 and the rotor 221 is larger than the distance in the axial direction between the yoke housing 213 (bottom portion 213 a) and the rotor is satisfied.

Fourth Embodiment

A fourth embodiment of a motor will be described below.

As illustrated in FIG. 19, a case 312 of a brushless motor 311 has a yoke housing 313 formed having a substantially cylindrical shape with a bottom and an end plate 314 for closing an opening of a front side (left side in FIG. 19) of the yoke housing 313. The yoke housing 313 is constituted by a magnetic body (iron, for example). The end plate 314 is constituted by a non-magnetic body (a resin material, for example).

As illustrated in FIG. 19, a stator 316 is fixed to an inner peripheral surface of the yoke housing 313. The stator 316 is provided with a stator core 317 having a plurality of teeth 317 a extending to an inside in the radial direction and a winding 320 wound around the teeth 317 a of the stator core 317 through an insulator 319. The stator 316 generates a rotating magnetic field when a driving current is supplied to the winding 320 from the external control circuit S.

As illustrated in FIG. 20, the stator core 317 has twelve teeth 317 a. Therefore, the number of slots 317 b formed between the teeth 317 a is also twelve.

As illustrated in FIG. 20, each of the teeth 317 a is provided with a winding portion 318 a and a protruding portion 318 b protruding to both sides in a peripheral direction from an end portion (tip end) inside in the radial direction of the winding portion 318 a. In the winding portion 318 a, U-phase, V-phase, and W-phase windings 320 are wound in a concentrated winding.

As illustrated in FIGS. 19 and 20, a rotor 321 of the brushless motor 311 has a rotating shaft 322 and is arranged inside the stator 316. The rotating shaft 322 is a non-magnetic body metal shaft and is supported rotatably by bearings 323 and 324 supported by a bottom portion 352 of the yoke housing 313 and an end plate 314.

As illustrated in FIGS. 21 and 22, the rotor 321 includes first and second rotor cores 331 and 332 and an annular magnet 333 as a field magnet interposed between the first rotor core 331 and the second rotor core 332 in the axial direction (direction along an axis L). The first and second rotor cores 331 and 332 are fixed to the rotating shaft 322 while an axial interval between each is held by press-fit of the rotating shaft 322. Moreover, the rotor 321 includes back-surface auxiliary magnets 334 and 335 and inter-pole magnets 336 and 337.

As illustrated in FIGS. 21 and 22, the first rotor core 331 has a substantially disc-shaped first core base 331 a and a plurality of (five in the fourth embodiment) first claw-shaped magnetic poles 331 b formed at equal intervals on an outer peripheral portion of the core base 331 a. Each of the first claw-shaped magnetic poles 331 b protrudes to an outside in the radial direction and also extends in the axial direction.

As illustrated in FIGS. 21 and 22, the second rotor core 332 has the same shape as that of the first rotor core 331. The second rotor core 332 has a substantially disc-shaped second core base 332 a and a plurality of second claw-shaped magnetic poles 332 b formed at equal intervals on an outer peripheral portion of the second core base 332 a. Each of the second claw-shaped magnetic poles 332 b protrudes to an outside in the radial direction and also extends in the axial direction. Then, the rotating shaft 322 is press-fitted into center holes of the first and second rotor cores 331 and 232. The first and second rotor cores 331 and 332 are fixed to the rotating shaft 322 so that a distance between outer surfaces in the axial direction of the first and second core bases 331 a and 332 a (distance between opposing side surfaces) becomes a certain distance set in advance. At this time, the second rotor core 332 is assembled to the first rotor core 331 so that the annular magnet 333 is arranged (sandwiched) between the first core base 331 a and the second core base 332 a in the axial direction. At this time, each of the second claw-shaped magnetic poles 332 b is arranged between the first claw-shaped magnetic poles 331 b adjacent to each other in the peripheral direction.

The annular magnet 333 is a magnet such as a ferrite magnet, a neodymium magnet or the like and is formed annularly having a center hole formed. The annular magnet 333 is magnetized in the axial direction so that the first claw-shaped magnetic pole 331 b functions as a first magnetic pole (N-pole in the fourth embodiment) and the second claw-shaped magnetic pole 332 b functions as a second magnetic pole (S-pole in the fourth embodiment). That is, the rotor 321 of the fourth embodiment is a rotor with a so-called Lundell structure using the annular magnet 333 as a field magnet. The rotor 321 is configured such that the four first claw-shaped magnetic poles 331 b to be the N-poles and the four second claw-shaped magnetic poles 332 b to be the S-poles are alternately arranged in the peripheral direction. The number of poles of the rotor 321 is set to eight (the number of pole pairs is four).

The first back-surface auxiliary magnet 334 is arranged between a back surface 331 c (surface inside in the radial direction) of each of the first claw-shaped magnetic poles 331 b and an outer peripheral surface 332 d of the second core base 332 a. The back-surface auxiliary magnet 334 has a substantially fan-shaped section in the direction orthogonal to the axis L. The back-surface auxiliary magnet 334 is magnetized such that a portion in contact with the back surface 331 c of the first claw-shaped magnetic pole 331 b becomes the N-pole which is the same pole as the first claw-shaped magnetic pole 331 b and a portion in contact with the outer peripheral surface 332 d of the second core base 332 a becomes the S-pole which is the same pole as its second core base 332 a.

Moreover, the second back-surface auxiliary magnet 335 is arranged between a back surface 332 c of each of the second claw-shaped magnetic poles 332 b and an outer peripheral surface 331 d of the first core base 331 a. The back-surface auxiliary magnet 335 has a fan-shaped section in the direction orthogonal to the axis L and is magnetized such that a portion in contact with the back surface 332 c becomes the S-pole and a portion in contact with the outer peripheral surface 331 d of the first core base 331 a becomes the N-pole. As the back-surface auxiliary magnets 334 and 335, a ferrite magnet, for example, can be used.

As illustrated in FIGS. 20 and 21, the inter-pole magnets 336 and 337 are arranged between the first claw-shaped magnetic pole 331 b and the second claw-shaped magnetic pole 332 b in the peripheral direction.

Moreover, as illustrated in FIG. 19, in the rotor 321, a sensor magnet 342 is provided through a substantially disc-shaped magnet fixing member 341. In more detail, the magnet fixing member 341 has a disc portion 341 b in which a boss portion 341 a is formed at a center and a cylinder portion 341 c extending cylindrically from an outer edge of this disc portion 341 b. To the magnet fixing member 341, the annular sensor magnet 342 is fixed so that it abuts against an inner peripheral surface of the cylinder portion 341 c and a surface of the disc portion 341 b. The magnet fixing member 341 is located at a position closer to the first rotor core 331, and the boss portion 341 a is externally fitted with the rotating shaft 322 and fixed to the rotating shaft 322.

Then, in the end plate 314, a hall IC 343 as a magnetic sensor is provided at a position opposite in the axial direction to the sensor magnet 342. The hall IC 343 outputs an H-level detection signal and an L-level detection signal to the control circuit S when it senses magnetic fields of the N-pole and the S-pole of the sensor magnet 342, respectively.

Subsequently, the yoke housing 313 of the fourth embodiment will be described in detail. The yoke housing 313 has a substantially cylindrically-shaped cylindrical portion 351 and the bottom portion 352 located on an end portion of the cylindrical portion 351 and is constituted to form a substantially cylindrical shape with a bottom.

The bottom portion 352 has a stator opposing portion 353, a rotor opposing portion 354, and a bearing accommodating portion 355.

The stator opposing portion 353 is a portion located inside in the radial direction of the cylindrical portion 351. The stator opposing portion 353 continues from an axial end portion of the cylindrical portion 351 and is opposed in the axial direction to the stator 316. The stator opposing portion 353 has an opposing surface 353 a opposed to the stator 316, a substantially annular groove portion 353 b formed in the opposing surface 353 a, and a surface 353 c on a side opposite to the opposing surface 353 a. The groove portion 353 b is opened toward the stator 316 opposing in the axial direction. At this time, the surface 353 c on the opposite side forms a planar state. That is, in the stator opposing portion 353, a thickness T1 at a spot in which the groove portion 353 b is formed is smaller than a thickness T2 at a spot where the groove portion 353 b is not formed. When the stator opposing portion 353 is cut in the axial direction, a sectional area of the spot in which the groove portion 353 b is formed is different from the sectional area of the spot where the groove portion 353 b is not formed. In more detail, a sectional area per unit length in the radial direction of the stator opposing portion 353 is different.

The rotor opposing portion 354 is a portion located inside in the radial direction from the stator opposing portion 353. The rotor opposing portion 354 is opposed in the axial direction to the rotor 321. Moreover, the rotor opposing portion 354 continues to an end portion inside in the radial direction of the stator opposing portion 353 through a cylindrical portion 357. Moreover, the cylindrical portion 357 extends toward an opening portion (end plate 314) of the cylindrical portion 351 from an end portion inside in the radial direction of the stator opposing portion 353. A tip end portion of the cylindrical portion 357 continues to an end portion outside in the radial direction of the rotor opposing portion 354. Thus, the rotor opposing portion 354 is located closer to the end plate 314 (opening portion of the yoke housing 313) in the axial direction than the stator opposing portion 353.

The rotor opposing portion 354 has an opposing surface 354 a opposed to the rotor 321 and a surface 354 c on the side opposite to the opposing surface 354 a. In the opposing surface 354 a, a substantially annular groove portion 354 b is formed. The groove portion 354 b is opened toward the rotor 321 opposed in the axial direction. At this time, the surface 354 c on the opposite side forms a planar state. That is, in the rotor opposing portion 354, a thickness T3 at a spot in which the groove portion 354 b is formed is smaller than a thickness T4 at a spot where the groove portion 354 b is not formed. When the rotor opposing portion 354 is cut in the axial direction, a sectional area of the spot in which the groove portion 354 b is formed is different from the sectional area of the spot where the groove portion 354 b is not formed. In more detail, a sectional area per unit length in the radial direction of the rotor opposing portion 354 is different.

The bearing accommodating portion 355 is a portion located inside in the radial direction from the rotor opposing portion 354. The bearing accommodating portion 355 forms a cylindrical shape with a bottom so as to accommodate the bearing 323.

Subsequently, an action of the brushless motor 311 configured as above will be described.

When a three-phase driving current is supplied to the winding 320 from the control circuit S, a rotating magnetic field is generated in the stator 316, and the rotor 321 is rotated/driven. At this time, rotation of the sensor magnet 342 faced with the hall IC 343 causes a level of the detection signal outputted from the hall IC 343 to switch in accordance with a rotation angle (position) of the rotor 321, and the three-phase driving current switched at an optimal timing is supplied from the control circuit S to the winding 320 on the basis of the detection signal. As a result, the rotating magnetic field is favorably generated, and the rotor 321 is continuously rotated/driven favorably.

Here, the yoke housing 313 of a magnetic body (made of iron) is located on one end surface side in the axial direction of the rotor 321, and the end plate 314 made of a resin is located on the other end surface side in the axial direction of the rotor 321 and thus, a part of the magnetic flux from the annular magnet 333 of the rotor 321 leaks to the case 312 (yoke housing 313), and there is a concern that magnetic balance is lost.

Thus, in the fourth embodiment, the groove portions 353 b and 354 b are provided in the bottom portion 352 (the stator opposing portion 353 and the rotor opposing portion 354) of the yoke housing 313 that can be a magnetic path of the leakage flux so as to reduce the sectional area of the bottom portion 352. In this configuration, the bottom portion 352 is configured such that the leakage flux is magnetically saturated easily by increasing magnetic resistance of the bottom portion 352.

Subsequently, advantages of the fourth embodiment will be described.

(8) The bottom portion 352 of the yoke housing 313 is configured such that a part of the bottom portion 352 has a sectional area different from the sectional area of the other portion in a section along the axial direction. As a result, magnetic resistance increases in a portion where the sectional area is small. Thus, the leakage flux is suppressed, and the output characteristics can be improved.

(9) A part of the bottom portion 352 has the groove portions 353 b and 354 b. As a result, magnetic resistance increases in a portion where the sectional area is small. Thus, the leakage flux is suppressed, and the output characteristics can be improved.

(10) The groove portion 354 b is formed at a portion (rotor opposing portion 354) through which the magnetic flux leaking from the rotor 321 having the annular magnet 333 generating a main magnetic flux to the bottom portion 352 of the yoke housing 313 and the magnetic flux leaking between the rotor 321 and the yoke housing 313 through the stator 316 both pass. Thus, the leakage flux can be reduced in advance at a spot where the leakage flux can concentrate easily.

(11) The groove portion 354 b is opened toward the rotor 321. Thus, the bottom portion 352 of the yoke housing 313 is spaced away from the rotor 321 at least at the position where the groove portion 354 b is formed. As a result, the magnetic flux leaking from the rotor 321 having the annular magnet 333 generating the magnetic flux to the bottom portion 352 of the yoke housing 313 can be reduced.

(12) The groove portions 353 b and 354 b are provided in the bottom portion 352 of the yoke housing 313 and formed annularly or preferably circularly. Thus, since the groove portions 353 b and 354 b are not interrupted in the peripheral direction, the leakage flux can be suppressed more reliably.

(13) The groove portion 353 b is formed at the position outside in the radial direction from the position opposed in the axial direction to the rotor 321 in the bottom portion 352. As a result, the magnetic flux leaking to the bottom portion 352 through the cylindrical portion 351 of the yoke housing 313 can be suppressed by the groove portion 353 b formed at the position outside in the radial direction from the position opposed in the axial direction to the rotor 321.

The fourth embodiment may be changed as follows.

In the fourth embodiment, the surfaces 353 c and 354 c on the side opposite to the opposing surfaces 353 a and 354 a on which the groove portions 353 b and 354 b are provided, respectively, do not have to have a planar shape.

As illustrated in FIG. 23, groove portions 358 and 359 may be provided by half blanking. In this case, portions corresponding to the groove portions 358 and 359 in the surfaces 353 c and 354 c on the opposite side have projecting surfaces 358 a and 359 a extending in the axial direction more than the surfaces 353 c and 354 c. At this time, similarly to the fourth embodiment, in the stator opposing portion 353, a thickness T5 at a spot in which the groove portion 358 is formed is smaller than a thickness T2 at a spot where the groove portion 358 is not formed. When the stator opposing portion 353 is cut in the axial direction, a sectional area of the spot in which the groove portion 358 is formed is different from the sectional area of the spot where the groove portion 358 is not formed. In more detail, a sectional area per unit length in the radial direction of the stator opposing portion 353 is different. Similarly, in the rotor opposing portion 354, a thickness T6 at a spot in which the groove portion 359 is formed is smaller than a thickness T4 at a spot where the groove portion 359 is not formed. When the rotor opposing portion 354 is cut in the axial direction, a sectional area of the spot in which the groove portion 359 is formed is different from the sectional area of the spot where the groove portion 359 is not formed. In more detail, a sectional area per unit length in the radial direction of the rotor opposing portion 354 is different.

In the fourth embodiment, each of the groove portions 353 b and 354 b does not have to be opened toward the stator 316 and the rotor 321 opposing in the axial direction, respectively. For example, each of the groove portions 353 b and 354 b may be formed on the surfaces 353 c and 354 c on the side opposite to the opposing surfaces 353 a and 354 a. Moreover, the groove portions may be formed on both of the opposing surfaces 353 a and 354 a and the surfaces 353 c and 354 c on the opposite side so that a part of the bottom portion 352 has a sectional area different from the sectional area of the other portion in the section along the axial direction.

In the fourth embodiment, the groove portions 353 b and 354 b do not have to be provided on both of the stator opposing portion 353 and the rotor opposing portion 354. For example, the groove portion may be provided on either one of the stator opposing portion 353 and the rotor opposing portion 354.

Moreover, the groove portions 353 b and 354 b do not have to be provided one each on both of the stator opposing portion 353 and the rotor opposing portion 354. For example, a plurality of groove portions may be provided at least on either one of the stator opposing portion 353 and the rotor opposing portion 354.

In the fourth embodiment, the groove portions 353 b and 354 b provided on the stator opposing portion 353 and the rotor opposing portion 354 do not have to be circular. Moreover, the groove portions 353 b and 354 b do not have to be annular. In essential, it is only necessary that the groove portions 353 b and 354 b can partially reduce a portion that can be a magnetic path of the leakage flux by functioning so that the sizes of the sections become different.

In the fourth embodiment, the groove portions 353 b and 354 b do not have to be provided on the stator opposing portion 353 and the rotor opposing portion 354. For example, the groove portion may be provided in the cylindrical portion 357 between the stator opposing portion 353 and the rotor opposing portion 354.

The groove portion may be provided in the cylindrical portion 351 in addition to the bottom portion 352.

The above-described first to fourth embodiments and each of the above-described other examples may be combined as appropriate. 

What is claimed is:
 1. A motor comprising: a stator having a stator core and a winding; a rotor having a first rotor core, a second rotor core, and a field magnet, each of the first and second rotor cores having a substantially disc-shaped core base and a plurality of claw-shaped magnetic poles provided on an outer peripheral portion of the core base along a peripheral direction, the field magnet being sandwiched between the first rotor core and the second rotor core in an axial direction, and the field magnet being configured such that the claw-shaped magnetic pole of the first rotor core and the claw-shaped magnetic pole of the second rotor core function as magnetic poles different from each other in a sandwiched state; a cylindrical case with a bottom, the case having a yoke housing that is a magnetic body and a lid portion closing an opening portion of the yoke housing, the yoke housing accommodating the stator and the rotor; and a plurality of back-surface magnet portions arranged in a gap formed on a back surface of the claw-shaped magnetic pole, wherein the plurality of back-surface magnet portions include a second back-surface magnet portion provided on the back surface of the claw-shaped magnetic pole of the second rotor core and a first back-surface magnet portion provided on the back surface of the claw-shaped magnetic pole of the first rotor core; and a size of the second back-surface magnet portion and a size of the first back-surface magnet portion are different from each other.
 2. The motor according to claim 1, wherein the core base of the first rotor core is arranged closer to the yoke housing in the axial direction than the core base of the second rotor core; and the second back-surface magnet portion has an axial length shorter than an axial length of the first back-surface magnet portion.
 3. The motor according to claim 2, wherein the claw-shaped magnetic pole of the first rotor core has an axial length longer than an axial length of the claw-shaped magnetic pole of the second rotor core.
 4. The motor according to claim 2, wherein a separation distance in the axial direction between a tip end surface of the claw-shaped magnetic pole of the second rotor core and the yoke housing is longer than a separation distance in the axial direction between an axial end surface of the core base of the first rotor core and the yoke housing.
 5. The motor according to claim 1, further comprising: an inter-pole magnet portion arranged in a gap formed between the claw-shaped magnetic pole of the first rotor core and the claw-shaped magnetic pole of the second rotor core in the peripheral direction, wherein the first back-surface magnet portion and the second back-surface magnet portion are formed integrally with the inter-pole magnet portion. 